The present invention relates to a cathode ray tube, and in particular to the shapes and form of a mask frame and an inner magnetic shield being joined together, designed to reduce negative effects caused by terrestrial magnetism.
FIG. 7 shows a conventional cathode ray tube (hereinafter referred to as a CRT) used in televisions and personal computers and the like. With a CRT such as the one indicated in this drawing, an image on the screen is produced when an electron beam 60 emitted from an electron gun is horizontally and vertically deflected by deflection coils 62 and then is scanned onto the entire display screen. During this process, if an external magnetic field, such as a geomagnetic field, is applied from a direction that is orthogonal to the direction in which the electron beam 60 is traveling, then the beam will become distorted in a manner such as that indicated by 61 (somewhat exaggerated in the drawing). This will result in mislandings of the beam, as it will fail to light the phosphor 64 on the panel 63 in the proper location. As a countermeasure to this, it is common practice to install into the interior of the CRT (in this case, the interior of the funnel section) an inner magnetic shield 65 that surrounds the scanning path of the electron beam.
However, it is impossible to entirely shield out an external magnetic field. Therefore, the actual role played by the inner magnetic shield 65 is that of (a) blocking out a certain amount of the external magnetic field, (b) changing the direction of the magnetic flux so that the electron beam is not affected, and (c) correcting the electron beam when it is affected by the magnetic line of force.
With a few exceptions, terrestrial magnetism is almost always the source of an external magnetic field. This terrestrial magnetism is broken down into a horizontal component (a vector component that is horizontal with the screen) and a vertical component (a vector component that is vertical to the screen). Of these two, it is well known that the vertical component does not pose a problem to the functioning of the CRT. This is due to the fact that the vertical component affects electron beam landing in a uniform way across almost the entire screen, and this phenomenon can easily be counteracted by using a correcting lens to ensure the proper formation of the phosphor surface so that it will compensate for the presence of the vertical component.
On the other hand, as shown in FIG. 8, the horizontal component 70 of terrestrial magnetism is more complicated in that its direction can change depending on its directional position relative to the CRT. Generally, this horizontal component 70 can be broken down into two directions: the CRT tube axial direction 71 and the CRT lateral direction 72.
Accordingly, when one considers a shield against terrestrial magnetism, one must ultimately consider the characteristics of a lateral magnetic field and a tube axial magnetic field, both of these forces being subcomponents of the horizontal component of terrestrial magnetism.
By measuring the amount of change that occurs in the phosphor surface beam landing when an external magnetic field equal to or more powerful than terrestrial magnetism is applied, it is possible to assess the various properties of the CRT. Points to be measured can be, among those shown in FIG. 9, the four corner sections of the screen and the upper and lower central sections of the length of the screen (hereinafter referred to as the NS sections), for example. It is particularly important that the following properties be noted: (1) The properties of the corner sections when a lateral magnetic field is applied (hereinafter referred to as xe2x80x9clateral cornerxe2x80x9d properties). (2) The properties of the NS sections when a tube axial magnetic field is applied (hereinafter referred to as the xe2x80x9ctube axial NSxe2x80x9d properties).
FIG. 10 shows the shape of the inner magnetic shield 65. As shown in FIG. 10, the inner magnetic shield 65 is a pyramid including two long sides 71 opposite to each other and two short sides 72 opposite to each other, where an opening 73 is formed at the top.
Each bottom of these sides is outwardly bent to form a bending edge, and in recent shadow masks to which tension is applied, the inner magnetic shield 65 is attached to the mask frame by fixing the bending edges to the mask frame.
As indicated in FIG. 11, the mask frame is composed of a pair of spanning members 81 (L-shaped in a sectional view) and a pair of U-shaped holding members 82. The pair of U-shaped holding members 82 are arranged opposite to each other. The pair of U-shaped holding members 82 are welded and fixed at two pairs of opposite ends of the pair of spanning members 81. A plurality of tensed wires are spanned between the pair of spanning members 81, the plurality of wires forming a shadow mask Ma. The plurality of wires are spanned at certain positions of the holding members 82 which are determined to hold the tension of the shadow mask Ma and to increase the strength of the frame in the direction of tension.
In recent years CRTs with larger screens and flat face plates have become the norm. In the case of CRTs with flat face plates in particular, the above-mentioned method of applying tension to a shadow mask is commonly used.
Conventional inner magnetic shields of CRTs that incorporate this method have tended to produce a dramatically higher amount or mislandings due to terrestrial magnetism. It is believed that this is because the magnetic properties of the shadow mask extremely change when it is placed under tension (Murai et al., SID2000DIGEST, pp582-585). For example, a conventional 25-inch CRT that has a lateral corner property of approximately 10 xcexcm and a tube axial NS property of approximately 10 xcexcm would degrade to have a lateral corner property of 15 xcexcm and a tube axial NS property of 30 xcexcm when tension is applied to the shadow mask.
Efforts have been made to improve the capabilities of inner magnetic shields constructed as in FIG. 10. For instance, the overall design includes a xe2x80x9cVxe2x80x9d-shaped section 74 that is cut out of the aforementioned short sides 72. The depth and width of the cuts have been optimized, but the amount of change in beam landing due to external magnetic fields equivalent to terrestrial magnetism have only been improved to:
(lateral corner property, tube axial NS property) (21 xcexcm, 23 xcexcm).
Additionally, the rates of change for the lateral corner properties and the tube axial NS properties ate roughly the same, with their measurements being in an inverse trade-off relationship, making it impossible to improve both properties at the same time.
An object of the present invention is to provide an inner magnetic shield that will solve the above problems and decrease the occurrence of drifted or uneven color throughout an entire display screen by decreasing the amount of mislandings caused by distorted electron beams occurring as a result of external magnetic fields such as terrestrial magnetism.
The present invention is a CRT designed to solve the above problems by being provided with an inner magnetic shield, a mask frame, a shadow mask that is affixed to the mask frame, and a face plate in which the mask frame and shadow mask are installed. The mask frame is made up of a pair of attachment members to which the shadow mask is attached, and a pair of positioning members that are joined with the attachment members to keep them in a predetermined position. The inner magnetic shield is of an outwardly square tube shape with a plurality or side surfaces to which at least one skirt is provided that extends, at a location where the inner magnetic shield is not in contact with either of the positioning members, to the vicinity of an attachment member. It is at this location that the skirt is magnetically coupled with the attachment member.
The present invention can also be a CRT designed to solve the above problems by being provided with an inner magnetic shield, a mask frame, a shadow mask that is affixed to the mask frame, and a face plate in which the mask frame and shadow mask are installed. In this case, the mask frame is made up of a pair of spanning members to which the shadow mask is attached in a high tension, and a pair of holding members that are joined with the spanning members to keep them in their proper position with the proper amount of tension place on the shadow mask. The inner magnetic shield is of an outwardly square tube shape with a plurality of side surfaces to which at least one skirt is provided-that extends, at a location where the inner magnetic shield is not in contact with either of the positioning members, to the vicinity of a spanning member. It is at this location that the skirt is magnetically coupled with the spanning member.
With this arrangement, the amount of mislandings caused by distorted electron beams occurring as a result of external magnetic fields such as terrestrial magnetism is lessened and the above-mentioned lateral corner properties and tube axial NS properties are improved. As a result, it is possible to decrease the amount of drifted and uneven color that occurs across the entire display screen.
Once such an arrangement is provided, it is desirable that the above-mentioned skirt be designed so that its length as it runs in the direction of the length of the spanning members is set so that any magnetic flux produced from an external magnetic field flows from the inner magnetic shield toward the mask frame.
In order that the inner magnetic shield can be mounted to the top of the holding members, the inner magnetic shield should be provided with a plurality of mounting surface members at the extreme ends of its above-mentioned side surfaces that extend toward the mask frame. And at locations where these mounting surface members are above the spanning members, it is possible for one or more skirts to be formed by being extended from the edge of the mounting surface members so that they reach the vicinity of a spanning member.
With this arrangement, it is desirable that the skirt that reaches the vicinity of the spanning member be designed so that its length as it runs in the direction of the length of the spanning member is xc2xc to ⅓ that of the distance between the holding members. This will result in a decrease in electron beam mislandings.
There is another possible design for this invention. As in the above descriptions, the present invention can be a CRT designed to solve the above problems by being provided with an inner magnetic shield, a mask frame, a shadow mask that is affixed to the mask frame, and a face plate in which the mask frame and shadow mask are installed. The mask frame is made up of a pair of attachment, members to which the shadow mask is attached, and a pair of positioning members that are joined with the attachment members to keep them in a predetermined position. The inner magnetic shield is of an outwardly rectangular tube shape with a plurality of side surfaces to which at least one primary skirt is provided that extends, at a location where the inner magnetic shield is not in contact with either of the positioning members, to the vicinity of an attachment member. It is at this juncture that this design varies from those described above in that it also includes at least one secondary skirt that extends down over the outer surface of the mask frame to cover an attachment member at the area where that attachment member is joined to a positioning member. The length of this secondary skirt as it runs in the direction of the length of the attachment members should be 1 to 2 times that of the area where the attachment member is joined to the positioning member. After these various arrangements have been made, the primary skirt and the secondary skirt are magnetically coupled with the attachment member.
The present invention can also be a CRT designed to solve the earlier-mentioned problems by being provided with an inner magnetic shield, a mask frame, a shadow mask that is affixed to the mask frame, and a face plate in which the mask frame and shadow mask are installed. The mask frame is made up of a pair of spanning members to which the shadow mask is attached in a state of tension, and a pair of holding members that are joined with the spanning members to keep them in their proper position with the proper amount of tension place on the shadow mask. The inner magnetic shield is of an outwardly rectangular tube shape with a plurality of side surfaces to which at least one primary skirt is provided that extends, at a location where the inner magnetic shield is not in contact with either of the positioning members, to the vicinity of a spanning member. Similar to that described in the above paragraph, this design includes at least one secondary skirt that extends down over the outer surface of the mask frame to cover a spanning member at the area where that spanning member is joined to a holding member. The length of this secondary skirt as it runs in the direction of the length of the spanning members should be 1 to 2 times that of the area where the spanning member is welded to the holding member. After these various arrangements have been made, the primary skirt and the secondary skirt are magnetically coupled with the spanning member.
With this arrangement, the amount of mislandings caused by distorted electron beams occurring as a result of external magnetic fields such as terrestrial magnetism is lessened and the above-mentioned lateral corner properties and tube axial NS properties are improved. Furthermore, covering with the secondary skirt the area where the spanning member and the holding member are welded together leads to the alignment of the magnetic flux and to a decrease in electron beam mislandings that occur near the welded area. This makes it possible to decrease the amount of drifted or uneven color that occurs across the entire display screen.
In order that the inner magnetic shield in this design can be attached to the top of the holding members, it can be provided with a plurality of attachment surfaces at the extreme ends of its above-mentioned side bending edges. And at locations where these attachment surfaces are above the spanning members, it is possible for the above-mentioned primary skirt and secondary skirt to be formed by being extended from the side bending edges so that they reach the vicinity of a spanning member.
With this arrangement, it is desirable that the primary skirt that reaches down to the vicinity of the spanning member be designed so that its length as it runs in the direction of the length of the spanning member is xc2xc to ⅓ that of the space between the holding members. This will result in a decrease in electron beam mislanding.
It is important to note here that the xe2x80x9cmagnetic couplingxe2x80x9d mentioned above indicates a smooth connecting process. This results in magnetic flux at border areas of the various elements that produce no mislandings or fewer mislandings in comparison to prior art.